Device for evaluating the sensitivity of an optical system utilizing the foucault knife edge test



Jan. 15, 1957 w. F. LINDSEY 2,777,

DEVICE FOR EVALUATING THE SENSITIVITY OF AN OPTICAL SYSTEM UTILIZING THE FOUCAULT KNIFE EDGE TEST Filed July 13, 1954 3) INVENTOR WAL m? E LINDSEY ATTORNEY United States Patent f DEVICE FOR EVALUATING THE SENSITIVITY OF AN OPTICAL SYSTEM UTILIZING THE FOU- CAULT KNIFE EDGE TEST Walter F. Lindsey, Hampton, Va.

Application July 13, 1954, Serial No. 443,174

11 Claims. (Cl. 8814) (Granted under Title 35, U. S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to apparatus for determination of the sensitivity of a schlieren optical system for use in the flow field of gases.

In the schlieren optical densitometer, use is made of an arrangement of a collecting lens deriving light from a source at its principal focus and passing parallel light rays through a condensing lens to a screen, which may take the form of a camera film. At the principal focus of the condensing lens an opaque knife-edge barrier, as devised by Foucault, is placed to obtain a control of the intensity of screen illumination. For an undisturbed state of the gas between lenses, a uniform screen illumination prevails. However, when the density in the air column between the lenses is disturbed, the light rays passing through the density disturbance are deviated and darker areas appear on the background screen surface due to interception of light rays.

The sensitivity of a schlieren densitometer may be dew termined by observing the displacement of the knife-edge necessary to bring the light intensity back to its original level, after a density disturbance. Experience has revealed that not only is such a displacement measurement diflicult to make but also it is impossible to determine the provide apparatus which will permit sensitivity determination of schlieren optical systems Without movement of the knife-edge. Objects also are to increase the precision and to simplify the method of determination of sensitivity in schlieren optical densitometers.

Other objects and features of the invention will become apparent on consideration of the following description of the invention, with reference to the accompanying drawings, in which:

Fig. 1 is a perspective view of a schematic set-up of the apparatus.

Fig. 2 is a plan view of the optical device used for producing the sensitivity measurements.

Fig. 3 is a section through the prism sector of Fig. 2.

Fig. 4 is a view of a part of the image obtained by the optical device of Fig. 2 when employed in the optical sys tem of Fig. 1, and

Fig. 5 is a view of a modification of the optical device.

As indicated in Fig. 1, in a wind tunnel a collecting lens transmits light rays from a source 11 through a 2,777,355 Patented Jan. 15, 1957 condensing lens 12 to a screen 13. At the principal focus 14 of lens 12 an opaque wedge 15 is positioned with its sharp edge at the focus so that it affords a partial block to all light passing through the principal focus and a complete block for all light rays deviated out from the focus that fall upon wedge 15.

At any convenient distance from the collecting lens a plane of observation is fixed at right angles to the main optical axis 16, which coincides with the principal axes of lenses 10 and 12, at which plane an optical device 17 is placed. This device, in Fig. 1, is shown as a flat opaque plate 18 having an aperture 19 centered on the axis 16 and two identical opposed prisms 20 and 21, the prism bases being outward and the prism apexes being inward relative to the optical axis, and the apex lines parallel with each other and with the upper blocking edge of wedge 15.

With the apparatus as described, it is apparent that when positioned in a wind tunnel with light source 11 at the principal focus of lens 10, parallel rays 22 and 23, with the prisms 20 and 21 removed, will come to focus at the cut-off edge of wedge 15 and thence form an image on screen 13. More specifically, parallel rays 22 and 23 after passage through condensing lens 12 will focus at the knife-edge of wedge 15 and continue to form a symmetrical light pattern on the screen 13, in accordance with the showing of broken lines 22 and 23. Should the prisms 20 and 21 be now placed as shown in Fig. 1, parallel rays 22 and 23 are bent by the prisms to form angled rays 22a and 2311. On refraction by lens 12, ray 22b, the continuation of ray 22a, is deviated from the cut-off edge of wedge 15 and falls on=the image plane without obstruction. Ray 23a, however, since it is also outwardly deviated, as at 23b, is blocked by the opaque wedge substance. Accordingly, an image 27 of prism 21 is formed upon the screen by a reduction in light intensity from the general illumination in image 25; similarly, an image 28 is formed of prism 20 by an increase in light intensity.

With the described apparatus in mind, it is apparent that the prisms function in a manner parallel to any transparent mass interposed in the optical flow system having a density differing from the general transmission medium. If, then, a group of these prism pairs be inserted in the light fiow area at the plane of observation, the prisms of each pair diifering in angle of light deviation over the others by a small angle, in progressive series, it may be possible to evaluate the sensitivity of the particular optical system involved.

Fig. 5 illustrates an optical device 30 which may be used for such sensitivity evaluation without movement of the Wedge 15. This device includes an elongated base block 31 of transparent material, such as glass, on which are mounted a series of pairs of identical prisms 32, positioned base to base along a median line 33. From end to end of the base 31 the angle of light deviation of the prisms of each pair differs over that of the preceding pair of prisms by a small amount, so that a gradation of angularity is secured from zero to a value in the neighborhood of sixty seconds. For example, in Fig. 5 the areas a and l have zero angularity, and the prisms of pairs b, c, d, e, f, g, h and k have an angularity of light deviation amounting in plus and minus second values to 1, 3, 6, 10, 20, 30, and 60, respectively.

In use, this optical device is mounted on the window ledge outside the test section of the wind tunnel, at the plane of observation of a schlieren optical system corresponding tol 7'inFig. '1. The device is here adjusted S'O that the "median line 33 is paralletto the knitewdge of the wedge 15 and the light rays from the source are incident, normally, on the bottom of base block 31. In addition to the normal parallel rays passing through the aperture, there will be rays through end blocks a and l of zero angularity and prism transmitted rays which will form a pattern on the screen along with the normal background. Depending on the sensitivity of the specific 6ptieal system, some of -the lower ptisms pairs will sltjow no'va'r'iativtin-inlight i'nten s'it'y 'be'tween the prisms of the pairs and the background illumination. However;moving along the prisrn 'p'atter-n toward pairs --6f .-greater angularityga pair is reaehed "Where -the upper pi ism intensity is darker, and the lower prism intens'i'ty is lighterfitlian the background intensity. This diverging 'pair together "with the next .-prece'ding pair "giving --un'iform i-nte'nsity determine the sensitivity range 'of I the opticalsystemfi the angularity lying between that :or "twonames V pairs. T he i'mag'es of end blocks a 'and 'l juxtaposejwith the pris'm in'iagej-images -of the background illumination: for oomiparison -purposes.

Th'ese relationships are brought out more 1 clearly 'in the modification of'the'optical device 40 as"dis'closd in -Fig. =2. In this figure, *a support 42, -designed for *mounting at the ledge of'window 44"at the'windtunnel tesfisection45 supports the prismsector 46.

The sector is formed of an opaque 'curvedplate' 48 of metal,- or the' l'ike, -in Which a plurality 'of areuately placed *openings- 50 areformed. These openings areiso place'd that' in themormal position of Fig-=2" one openingis di- 'rectly-ab'ove another toformlpairs, there being single -termi-nal-openings. Each ofthese openings iscircular and 'as' shown inFig. 3, has an 'annular ledge =51 on which "the circular. prisms 52 are placed, there-'be'ing aretaining ring- 53' for. each prism.

It is pointed out that, with theexception of the" terminal -prisms, the.p'risms are distributed in vertical pairs, 'as shown in Fig. 3, with the bases adjoining to -secure opposite light- "deviation as indicated by' broken lines 54 of this 1 figure. In Fig. 2 thesea pairs are lettered fromde'ft to right,-as'm; n, '0, p, :q; r, s, and t, the terminaL-prisms'being lettered u 'and' v. The prisms of each pairare identicalin angularity but each succeeding pair from left to right,-as in-Fig. 2, has increasing angularity with u and v values equal to zero. 'For example, a usable angle sequence -is splus and minus 3,6, 10, 15, 20, 30,40 and-'60 seconds' for .pairs,- m, n, 0, p, q, r, s,-and 1; respectively. The, plane 7 containing the. prisms is parallel to a plane including the knife-edge of wedge 15. In order tofacilitet'teadjustment -of the sector 46 for, proper. prism: positioning a-= straight I and vertical orientation wire 55 is securedtoonadjacent Abe-window 44 anda small cylindrical-aperture 47-of -aboutone sixteenth 'inchdiameteris formed in the sector "46.

In sensitivity determination with this modified-optical -dev-ice40, the device is positioned at the plane 'of observation so that-aliner'joining similan-points-in corresponding end prisms, 'for example-u and v, in- Fig. 2, is-.parallel to the knife-edge of wedge 15oz perpendicular to :the wire line 55- with the wire- 55=at right =anglesto; the;l-:nife-edge i of wedge 15. The sector 46is so-oriented-thatlight from source-11 afterpassage-through lens- 10 and refiection from the plane rear surfaces of. prisms 52' forms :images on"lens- 10 atpositions corresponding to the; positiomof the sectorin the planeof observation. At this.pos'ition light visible throughaperture 47 should-have-awircular boundary. It elliptical, the sector s hould -beorientedin the directionof the-short diameter of the light ellipse -to obtain a circular form, \vhen the adjustment is correct for instrument measurement. Parallel rays fromsource ll are now passed through sectorp'fisms and tunnel window 'd-fp'roducing on the screen 13 an'image pattern of. the prism rays'and' backgroundillumination. Asi11thepattern of the'device'so'of Fig. 5,a prisrn'pairinay be found,

as p, where the light intensity is greater than the background "intensity, "in the lower image, and lesser in the upper image, With the next preceding pair images of intensity value approximately equal to each other and to 5 the background value. The degree value of angular deviation is thus determined as between the degree values of these two prism pairs, which in this case would be between and seconds.

If greater sensitivity measurement is desired, an optical l0 device is used which is identical to 40 of Fig. 2, but having a variation in prism a'ngleuof deviation from 0 to seconds instead of from 0 to 60 seconds. Using this 'finer gradation 'device thelO to 15- seconds ofJthe'coarser gradation device "may be definitely' fixed at I 15 seconds.

In Fig. 4, by stippling'theimage areas, the fiect of increasing prismrpairaian'gularity.is.indicated toward the right with equivalent image effects on theleft, where the angleof deviation istoo smallto be afiectedby the given optical system.

Obviously, modifications other than described may be made, -and"it is, therfore, to "-beunderstood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is"c1aimed"is:

1. Optical sensitivity "determination "apparatus foruse i-nsclilieren type optical systems comprising collecting and condensing lensesand includinga-gas test section,"a

light-sou'rceyan'image screen,'-a Foucault'knife-edge inter- -"ce'ptor -betweensaid screen and condensing lens "at the principal focus of said condensing lens, anelongated support adapted 'for placement between the lenses of'and eaten-1:211--to" 'the "gas '-test section andtransverse to the 1 optical axis at said schlierensystem, and a plurality of uxtaposed pairs of prisms-forming-a line" of prismsfithe ='p'risms ofeach pairbeing-identieal-in form -butopp0sedin singularity, mounted on said support, the planes of light dev'iation bf the prism pairs -being= parallel and the prism an'gle-=ef each pairchanging progressively from one sup- 'po'rt end to thedther.

2.' The optical sensitivity determination'apparatus -as defind in claimlwith separateprisms of-each pair being placed in' separate -arcuate lines; the prisms of one 'line b'e'ing formed to transmit rays-'of *pos'itivewangularity and the prism of the other line being formed-to transmit rays at negative angularity.

A -The optical-sensitivity--determinatiom apparatus as 'de'fined -in claim-Z, Withan added prisrn' at either-end of said p'risin' line'havingzeroangularity.

4. 'l he optical sensitivity I determination apparatus as defined in 'claim l with the angles of light deviation'of the an'glesbfipairs next adjoining.

5. The optical sensitivity determination apparatusas defined in claim 1 with' the angle of =1ight deviation of the j prisms of= each -pair being opposed and differing cumu- I atively ov'er 'the angle of the prisms in' the next pair,

tn'oving' from pairs ofi low angle to' I the 'pair of largest angle.

6. The optical sensitivitydetermination apparatus as defined inclaim 1with-=said pairs of prisms" being posi- "'"tiond' side"byside-alonga'straight'line, and a transpar- 'entbasefor supporting'the'prisms on said' lensoptical axis between said lenses and transversely to said axis.

7f The 'optical- 'sensitivity'determination apparatus as defined in claim 4 with the terminal pair of prismsin raid "'Iine of-prismshaving'zero angularity.

8. An optical device "for "determination of"sensitiv'ity jin -"schlierenatype optical apparatus using the 'Foucault knife-edgelight"interceptor which "comprises a upport,

a-seetor plate *atta ched 'to said support 'having'arcuately placed openings therein placed one above anotherrin'p'airs, opposed identical prismsinthe' two openings of eaclrpair, I e"angle" of light deviation of the prisms'bf each pair 7 5 differing from that of 'zidjacenfpairs, and'meansfor'adjusting said device with reference to the Foucault knifeedge when placed in said optical apparatus.

9. The optical device as defined in claim 8 said adjusting means including a fixed aligning Wire adjacent said sector and an orientation apertune in said sector having an axis normal to the sector plane.

10. The optical device as defined in claim 8 with the angle of each prism of each pair changing by progressive amounts, moving from one end of the sector to the other.

11. An optical device for determination of sensitivity in optical apparatus using the Foucault knife-edge light interceptor Which comprises a support, a transparent base block of elongated form, a series of pairs of prisms fixed base to base along the long axis of said block, the prisms of each pair being identical and the light deviation angle of the prisms of each pair differing progressively, moving from end to end along the block axis.

References Cited in the file of this patent UNITED STATES PATENTS 

